This invention pertains to RF and fiber-optic architecture of HFC (Hybrid Fiber Coaxial cable) cable television or cable-like network systems and two-way digital communications to cable modem and digital settop units.
HFC Networks
FIGS. 1a, 1b show a generic HFC network. FIG. 1a shows the head-end whereas FIG. 1b shows the entire network.
The Head-end (HE) 8 contains the equipment respectively 10, 12 and 14 that receives the analog and digital television signals and the digital data signals from multiple local and remote sources (satellites, off-air sources, terrestrial microwave, local tape systems, local video servers, computer servers, IP routers) and conditions these signals for transmission to the home terminals (HT). The home terminals 18-1, 18-2, etc., are analog and digital video xe2x80x9csettopsxe2x80x9d (cable TV set top boxes) and digital cable modems. The head end 8 also receives the reverse (upstream) transmissions from the home terminals and processes them, in coordination with the downstream transmissions and the input/output signals from/to outside digital networks. The equipment to perform this transceiving function at the HE with respect to the home devices and the core data networks connected to the HE is called in this disclosure Interactive Termination System (ITS).
Consider first the downstream transmission of analog video, then the transmission of digital video and data. Analog video is transmitted downstream by FDM (Frequency Division Multiplexing), whereby a composite spectrum consisting of multiple analog channels is generated by RF (radio frequency) combining the output of analog modulators 20-1, 20-2, etc., each of which is driven by a baseband or IF analog video channel. The composite FDM signal is then applied to one or more linear analog laser transmitters 24-1, 24-2, 24-3, etc., and transmitted over a xe2x80x9ctreexe2x80x9d structure 28 of optical fiber to the fiber nodes 30-1, 30-2, etc., where conversion from optical to RF format occurs at optical receiver 29.
From the fiber nodes the signals are distributed to the home terminals via a tree and branch structure 40 consisting of coaxial cables with amplifiers 42-1, 42-2, etc., periodically placed to make up for the signal loss, taps and coaxial drops to the subscriber units HT. Each coaxial cable drop terminates in an RF set-top converter HT which bandpass selects a particular analog television channel out of the composite spectrum.
The band from 550 MHz to 750 MHz is used for downstream digital transmission from digital TV sources 12. Digital QAM (quadrature amplitude modulation) modulators 50-1, 50-2, 50-3, etc., are used to map multiple streams, each of several tens of Mbps into 6 MHz channels. The modulators 50-1, etc., are positioned at the transmit side of a digital link, which runs over an analog linear medium, such as the HFC medium. The digital input to each QAM modulator at the HE is typically an MPEG-2 multiplexed digital signal carrying multiple digital video MPEG-2 programs and/or data channels from multiplexes 52-1, 52-2, 52-3, etc.
The digital video inputs to the multiplexers 52-1, 52-2, etc., are generated by digital video encoders in sources 12 each of which digitizes and compresses an analog video input signal. Alternatively the digital video input signals originate from digital video servers or are received from remote sources via satellite. On the transmit side, an RF summing/splitting matrix 53 combines the RF signals carrying analog TV, digital TV, and data signals; the data signals are provided from the ITS via modulators 55-1, 55-2, etc.
The coaxial cable path is used for return (upstream) as well as for forward (downstream) transmission. The 5 to 42 MHz band (used in the US) and the corresponding range in international cable systems, called here the lowband, is dedicated to the upstream transmission. Home terminals HT such as cable modems and interactive digital video settops, in addition to receiving downstream digital transmissions by means of their QAM demodulators, also have the ability to map their digital return transmissions onto RF waveforms using upstream burst transmitter modulators. Modulation formats such as QPSK or 16-QAM are typically used, however in the return path the transmission is not of a continuous bitstream as in downstream but rather occurs in bursts of short packets of data randomly occurring in time. The data bursts at the home terminals HT are encoded into short sequences of symbols by the QPSK or 16-QAM burst transmitter modulator (called here a burst transmitter). After upstream propagation all the way up to the head-end 8, these bursts are converted by QPSK or 16-QAM burst receiver demodulator (called here a burst receiver) into the original data packets. This process which is called detection occurs in the front end of the ITS 56-1, 56-2. Several burst receivers may be used with the corresponding number of upstream channels, each receiving packets over a single upstream channel frequency. Each upstream channel frequency may be shared by many home terminals, by TDMA (Time-Division Multiple Access) as arbitrated by the ITS at the head-end.
In the return path, RF return signals from the home terminals HT propagate back towards the head-end 8, going back up the drop, the tap in tree 40, and back through the amplifiers 42-1, etc., which have bidirectional capabilities to support the return path. When the return signal reaches the fiber node 30-4 it is diplexed by diplexer 59 (i.e., directed on a separate upstream path based on the orthogonality of the upstream and downstream frequency bands) amplified and applied to a return optical transmitter 58 and transmitted back up to the head-end, typically on a separate optical fiber than the one used for downstream transmission.
At the head-end 8 the return signal is photodetected in a return-path-receiver 60-1, 60-2, i.e. converted back to electrical (RF) form, and is then split at RF summing/splitting matrix 68 and fed to analog or digital receivers for the various return service applications, in particular, it is input into the return path demodulators 64-1, 64-2.
High performance broadband HFC networks are essentially broadcast networks. To increase the capacity, a combination of a digital switching network and a multiplicity of smaller scale broadcasting subsystems can be employed, whereby the subscriber population is partitioned into multiple sets, with each set of subscribers being allocated one switched bi-directional digital data stream, such that different streams belonging to different subscriber sets are generally independent. This narrowcasting architecture essentially consists of a master switched system or network, with the switch ports driving smaller scale HFC broadcast, each addressing a serving area of a few tens or hundreds of subscribers called the narrowcasting domain. Domain-specific digital content, namely , two-way interactive data and interactive digital video, namely VOD (Video-On-Demand) is routed or switched to/from each domain via the HE or multiple hubs. One can differentiate between downstream and upstream narrowcast domains.
As subscriber penetration increases it is necessary to develop methods to efficiently concentrate the return path signals from large number of subscribers all the way to the head-end, while maintaining small upstream narrowcast domains. This is done by segmenting the HFC system into a larger number of return path domains, associating a smaller number of subscribers with each return path transmitter at the node. This is beneficial with respect to the ingress noise accumulation but also increases the upstream bandwidth per subscriber.
The relationship between downstream and upstream narrowcast domains is as follows. The relevant factors in determining the optimal ratio of the upstream and downstream domain sizes are digital transport capacity in the downstream and upstream (a function of the spectra available and the spectral efficiency), and the asymmetry profiles in the upstream vs downstream average transmission rates required per subscriber. For example, it is given that only 37 MHz of bandwidth are available in the downstream and the modulation scheme there (QPSK or 16-QAM) is less spectrally efficient than in the upstream (QAM-64 to QAM-256). Since more bandwidth is available downstream than upstream (though this depends on the utilization of the downstream bandwidth between analog and digital broadcast vs narrowcast), and since the traffic is asymmetric with more bandwidth required downstream, it follows that the downstream narrowcast domains should be larger than the upstream narrowcast domains. It is desired to have the size of the downstream and upstream narrowcast domains decoupled. One way of manipulating the size of narrowcast domains is to design the system in the first place with small and equal downstream/upstream domains and to RF combine the upstream and downstream transmissions.
Once the digital capacity of the fixed return path portion of the spectrum (e.g. 5 to 42 MHz in the US, with each 3.2 MHz carrying 5 Mbps in QPSK and 10 Mbps in QAM16 modulation) is divided among fewer subscribers, not only does the bandwidth per subscriber increase, but also the lower noise may allow using more spectrally efficient modulation schemes such as QAM16 rather than QPSK. The challenge is to reconcile this with the trend of eliminating switching equipment (ITS and video on demand VOD) from the hubs and bringing all the returns from tens of thousands of subscribers back to a master head-end. One must then collect return signals from a minimal number of RF branches, while devising methods to transmit those returns in some efficient multiplexed way back to the head-end.
Known alternatives for a high performance broadband return path include space-division multiplexing using multiple fiber return paths; frequency stacking using an FDM return path; dense wavelength division mutiplexing at an optical fiber node in the return path; time division multiplexing in the return path; and PCM (pulse code modulation) for a digital return path.
Dail U.S. Pat. No. 5,878,325, discloses shifting the point of detection of the analog waveforms which encode the upstream digital communication, away from the head-end and into a fiber node (where optical RF conversion takes place).
In an example of partitioning the 2000 homes node into four return paths each covering 500 homes, four groups of return demodulators may be used in the fiber node, each group listening to the FDM channels allocated on each of the four RF coaxial cable legs. The digital outputs from these return demodulators are Time Division Multiplexed at the node into a single digital stream. A digital baseband link running over fiber optics should then be provided from the fiber node to the head-end.
This is very efficient. If QPSK is used then the spectral efficiency is approximately 1.5 bits/sec per Hz, i.e. less than 1.5 B bits are required to carry the upstream QPSK transmissions partially filling a return bandwidth of B Hz.
Multiple Access Arbitration in Digital HFC Networks
HFC media are broadcast, such that transmissions from the head-end are received by every HT, and conversely return transmissions from the HTs may superimpose upon one another when they reach the head-end.
The most natural design for the multiple access in cable systems uses centralized management from the head-end, which becomes the xe2x80x9cmasterxe2x80x9d, with the home terminals HT becoming xe2x80x9cslavesxe2x80x9d, in the sense that the master decides when a slave is allowed to send data. The head-end is then a single point of coordination.
The critical layer for interactive communications with HTs such as cable modems and settops is the head-end MAC (Medium Access Control) layer, used to arbitrate access of multiple users. This layer is located between the physical layer and the higher application-oriented layers. There are multiple choices for the implementation of the MAC layer. Given that the propagation delays from the head-end to the home terminals are typically much larger than the durations of packets, it follows that distributed contention based MAC schemes such as ALOHA or CSMA/CD (Ethernet) would be very inefficient. Instead a class of protocols which are called here reservation based limited contention protocols are used.
These protocols combine the best properties of collision-free and contention-free protocols, i.e. using contention at low loads to provide low delay, while using a collision-free polling technique at high load to provide high throughput.
In particular, the preferred choices for the MAC implementation for HFC systems are Reservation Based Limited Contention Hybrid FDMA/Slotted-TDMA HFC schemes, referred to here as RBLC-HFC schemes.
Relevant established standards (or standards in the making) are DOCSIS, DVB-RC, DAVIC, IEEE 802.14, and OpenCable. The most widespread and mature standards are the first two. The DOCSIS standard was originally called MCNS and the DVB-RC standard was adopted from a prior standard called DAVIC (Digital AudioVisual Council).
The IEEE 802.14 standard seems to be failing to become widely adopted. Finally, a new North American standard for set-tops called OpenCable is in the making, and is likely to include the DAVIC and DOCSIS standards as a subset.
Therefore this disclosure focuses on the DOCSIS and DVB standards as the most widespread, and best exemplifying the generic features of all standards for broadband communication over HFC networks, that are based on the RBLC-HFC protocol, whereby the home terminals use a combination of reservation and contention techniques to establish broadband bi-directional communications with the head-end. These standards support different access modes for upstream data transmission.
While details of these the two main DOCSIS and DVB standards differ, for the purposes of this disclosure, the systems are described using a generalized nomenclature pertaining to both.
The Head-End ITS (Interactive Termination System) refers here to both CMTS (Cable Modem Termination System) and INA (Interactive Network Adapters) Head-End (HE) controllers which implement the MAC layer as well as the higher level application layers and the physical layer at the head-end.
DOCSIS Standard
The MAC frame formats for the DOCSIS standard are set forth in that standard, incorporated herein by reference. The following is a brief description of principles of operation of the standard.
The upstream channel is modeled as a stream of mini-slots along the time axis, the time reference for which is generated by the CMTS and communicated to all CMs. The CMTS arbitrates access to these slots by each of the cable modems. For example it may grant some number of contiguous slots to a CM for it to transmit some data or it may assign a number of slots for contention among stations that wish to transmit some data without having made prior reservations or it may allocate mini-slots to stations that communicated to the CMTS their wish to be allocated reserved bandwidth.
The DOCSIS MAC protocol governs the requesting, granting and using upstream bandwidth. An initial ranging and calibration procedure is first run such that new CMs be able to join the link and time their transmissions so that the CMTS receives these transmissions in the time reference intended by it.
DAVIC-RC Standard
There are two variants of the DVB-RC standard, the IB (in band) and OOB (out of band) methods. In both methods a QPSK return path is provided from the NIU (Network Interface Unit) which is the relevant subsystem in the home terminals, namely digital video settops and cable modems abiding by the DVB standard. The two variants differ in the provision of the interactive return path. Under the IB method the downstream control channel is transmitted over a QAM channel which can also multiplex digital video traffic. Under the OOB method a dedicated QPSK downstream channel is used for control and synchronization of the home terminals.
Interactive Termination System
FIG. 2 illustrates operation of a conventional ITS 56 system in the HE (head-end). The description of the internal structure of the ITS is generic, covering the common aspects of the DOCSIS and DVB-RC standards. FIG. 3 presents the internal structure of the INA in particular.
The timebase counter 80 is driven by an HE local clock (not shown) and synchronizes all the home terminals. In both standards timebase counter 80 actually generates two clock outputs, related to each other by a fixed integer ratio. The two related counter outputs are the sync count signal 82 and the scheduler count signal 84, such that a fixed number of counts of the scheduler count corresponds to a single count of the sync count.
In the DOCSIS standard the sync count is a 32 bit count driven by a 10.24 MHz clock, whereas the schedule count represents 6.25 xcexcsec/64 divisions.
In the DVB-RC standard the sync count represents the count of ESFs (extended superframes) whereas the scheduler count is a finer count describing a finer subdivision of the duration of the extended superframe into regular time ticks. In the DVB standard a T1 type of framing, namely the extended superframe, is used for embedding payload and downstream overhead messages.
The sync count 82 is periodically embedded within downstream data stream. The downstream frame including the overheads and the sync message or sync bits is constructed by means of the Frame Mapper module 88, which in turns drives the physical layer modulator 90. In the DOCSIS standard the timestamps are embedded in separate SYNC messages, whereas in the DVB-RC standard the Sync count bits are embedded within the M1-M10 overhead bits of the extended superframes transmitted downstream which contain the actual payload as well as other encapsulated overheads called MAC flags.
The scheduler count 84 optionally is modified by a fixed calibration offset loaded in an offset accumulator 92 (supplied from CPU microprocessor 96) via adder 98 and then the resulting running time reading is compared by comparator 100 with the contents of a next arrival register 102 which indicates the expected time for the next arrival of a burst from one of the home terminals.
The values of the next arrival register are successively loaded from a queue 104 in the FIFO memory of the ITS processor 96. This queue 104 is generically called here schedule.
The schedule 104 is a data structure generated at the HE to be transmitted to all the HTs over a schedule_message. It indicates how to assign the time axis to the various HTs, by partitioning it into time intervals, each starting at the planned arrival time of packets or bursts from each of the HTs.
The schedule 104 consists of an ordered array of times when an arrival is due at the HE burst receiver (for each upstream channel and its associated burst receiver(s) one such schedule is maintained). It is communicated downstream by a related schedule_message, from which each HT may reconstruct a subset of the schedule containing an array of planned arrival times of bursts from that particular receiver. In the DOCSIS standard the schedule_message is called allocation_Map while in the DVB standard, the schedule_message consists of bits interspersed in the overhead of the extended superframe, namely the b0 ranging slot indicator and b1-b6 slot boundary definition MAC flags, as well as transmit opportunity assignments communicated in certain MAC messages such as the Connect_Message and the MAC_Reservation_Grant_Message.
When the scheduler running time count becomes equal to the contents of the next arrival register 102, a trigger pulse is generated by comparator 100 to indicate to a control input of the burst receiver 106 the expected time of arrival of the next burst.
In the reservation modes of operation the schedule 104 communicated to the HTs allows effective generation of a polling behavior, whereby each of the HTs in turn, in the order and at the timing determined by the structure of the schedule, transmit their packets in sequence. It is the responsibility of each HT to start transmitting at the proper allocated time (called transmit opportunity) as defined in the schedule. It is the responsibility of the HE to transmit a consistent schedule ahead of the intended transmission times for all the HTs. If all the HTs follow this procedure then it is ensured that only one HT transmits at a time and collisions are avoided while the available time for transmission is optimally utilized, without leaving idle intervals.
In order for this procedure to work, the arrivals of packets from the HTs to the HE must be first synchronized. To this end, upon initialization, a ranging protocol is executed insuring that the various delays from the HE to the home terminals are calibrated out by applying appropriate offsets to the local clocks of each HT. Once the ranging process is completed, time events indicated in the schedule may be interpreted in a literal way as both transmission times (according to each HT""s local clock) as well as arrival times at the HE of the respective HT transmissions. It is then guaranteed that once the schedule indicates that a particular HT should transmit a packet at a certain (local) time To then that packet indeed arrives at the HE at the same nominal time value To (but referred to the HE clock). Therefore the schedule indicates the intended times of arrival of each packet at the head end, which, if literally interpreted by the home terminals as burst transmission times, will then coincide with the intended arrival times.
In the reservation mode the ITS assigns different home terminals to disjoint time intervals so that there are no collisions between the various transmissions. In the contention mode of operation, collisions are allowed, as certain time intervals are allocated to the simultaneous transmission of multiple home terminals. Contention is resolved by a back-off procedure whereby HTs randomly delay their to subsequent contention intervals to later times within the same contention interval.
The ranging procedure during initialization is itself executed in a contention mode, since more than one modem may attempt to join in by sending a message called RNG_REQ (Ranging Request) in the DOCSIS standard and Sign-on_Response in the DVB-RC standard. This message is generically called upstream_probe to cover both standards. The duration of the ranging interval is taken sufficiently long to allow for the differences in propagation time from the HTs to the HE. At this point, due to unknown propagation delays, the actual arrivals of the probe_message bursts from the HTs to the HE do not generally correspond to the head-end burst receiver trigger times which in turn correspond to the expected times of arrival as defined the schedule. However, the offset between actual and expected arrival times may be determined by the burst receiver which must have the capability to measure the time difference between the expected arrival as signaled by the burst receiver trigger and the actual arrival of the burst, and communicate this time offset to the ITS processor (CPU) 96.
This timing difference called timing_offset is equal to the total round trip propagation and processing delay, and stems from the delayed acquisition of the sync counter clock by the HTs as well as the delay between transmission of bursts at the HTs and their reception at the HE due to propagation and processing delays.
To complete the ranging process, once the timing_offset is measured by the HE burst receiver, it is communicated back to the HT by an overhead message which is called in the DOCSIS standard RNG_RSP (Ranging Response), while in the DVB-RC standard it is called Ranging_and_Calibration_Request message. Generically call this message downstream_calibration message here.
Each HT undergoing the ranging process is now able to correct its transmission clock accordingly by an amount equal to the timing_offset. This insures that future bursts transmitted by each HT now arrive at the ITS burst receiver precisely at the corresponding times of the receive triggers as derived from the schedule for each of the HT entries.
In addition to the timing trigger, the burst receiver 106 is also fed with estimates of the expected RF power for the next burst as well as the expected frequency of the next burst and it optionally (only in DOCSIS) measures the frequency response characteristics of the upstream channels for subsequent correction of equalization of the burst transmitter filter coefficients in the HT.
This necessitates the burst receiver to be equipped with the additional capabilities of a sophisticated measurement device for the ratio (difference on a decibel scale) of received RF power and expected RF power of the burst as well as for small frequency offsets between the intended and actual frequency of reception and the frequency response of the channel. These measurements, which together with the timing offsets are collectively called here offset_attributes, should also be communicated by the RNG_RSP to the HT in addition to the timing offset. Upon receipt of these offset_attributes the HT should correct the corresponding absolute parameters (time, power, frequency, equalization coefficients) by the received offset amounts, such that after the correction, a repetition of the measurement of these parameters by the burst receiver at the HE would eventually yield zero offsets. This procedure is performed at least once upon initialization or reset of the ITS and HT until the offset_attributes converge to zero, but it should also be repeated every once in a while to counteract the offset_attributes slowly drifting away from their null values due environmental effects.
Burst buffer 108 receives the output signal from burst receiver 106 and couples it back to CPU 96, which in turn outputs the payload and MAC messages to the frame mapper 88.
FIG. 3 shows the INA for the DVB-RC standard, and is in many respects the same as FIG. 2. In FIG. 3, the timing circuitry is somewhat different, including a local clock 116 driving divider 118 and timebase counter 120. ESF counter 124 (ESF is DVB-RC nomenclature) couples to ESF frame mapper 128, as does buffer 132.
Home Terminals
A generic home terminal (HT) 18 is shown in FIG. 4 which applies to both standards. FIG. 5 shows a DVB-RC version of the FIG. 4 apparatus.
The frame recovery and parsing module 132 separates the incoming stream from demodulator 134 into substreams of payload bits and MAC overhead messages 136 as well as the sync timestamps 138 which are fed into a timestamp register 142 as they arrive (the timestamp register is called Upstream Position Register 143 in the DVB case).
The timebase recovery module 144 is a digital phase locked loop driving a local counter which tracks the samples of the timestamp register, smoothing any random rapid variable delays that may have been superimposed on the timestamp register readings along the way. The recovered count is called here local timebase. It lags the sync count timebase at the HE 82 by an amount of time equal to the propagation delay plus any processing delay in the frame and timebase recovery modules.
The local timebase is next additionally offset at adder 148 by a value stored in the offset accumulator 150 (from CPU 152), then it is compared with the reading of the next departure register 154, which is in turn loaded with the transmit opportunities times for the particular HT, as extracted from the schedule queue stored in memory 156. The schedule is extracted in advance out of appropriate overhead messages transmitted over the downstream broadcast channel by the HE to all associated HTs.
When the local timebase count becomes equal to the value stored in the next departure register, a pulse is generated in the comparator 160 and applied to the burst transmitter 162 to trigger the transmission of the next burst which was stored in buffer 166.
Subsequently, the next departure register 154 is updated by retrieving the next value out of the schedule queue 156, corresponding to transmission (and arrival) times of bursts from the particular HT. After the next time of transmission is loaded into the next departure register, the process is repeated.
The transmit and receive triggers at the HT and HE respectively, both occur when the running counts of the timebases at the respective locations become equal to the nominal value To as specified in the schedule queue for the particular burst of the particular HT, but after ranging calibration the actual times when these two triggers tagged by the same nominal time To occur, are in fact different such that the burst triggered at the HT by the trigger at local time To, is assured to arrive to the burst receiver at the HE concurrent with its burst receiver trigger, also at nominal time To but measured with respect to the HE.
To grasp the timing relationships, notice that prior to execution of the ranging procedure the burst transmit trigger lags the burst receiver trigger by the total one-way delay. This means that the probe burst sent by the HT during ranging arrives at the HE burst receiver when it lags by the total round trip delay (with respect to the burst receiver pulse).
Advancing the burst transmit pulse by the round trip delay (twice the one-way delay) causes now the arrival at the pulse concurrent with the burst receive trigger at the HE.
This means that after ranging the burst transmit trigger at the HT now precisely advances the burst receiver trigger by an amount equal to the propagation delay from the HT to the HE plus any burst transmitter processing delay (total one-way delay).
FIG. 9 shows a DVB-RC version of the FIG. 8 home terminal, with similar elements similarly labeled. Here the demodulator 170 is a QPSK demodulator driving the ESF frame recovery module 172.
This invention pertains to broadband HFC distribution systems to digital home terminals such as digital settops or cable modems, using bi-directional transmission standards such as DOCSIS and DVB, or more generally using any multiple access scheme characterized herein as of the Reservation Based Limited Contention Hybrid FDMA/Slotted-TDMA HFC (RBLC-HFC) type as described above. This allows application of the invention to future RBLC-HFC standards in addition to the existing DOCSIS, DVB-RC and IEEE 802.14 standards.
In conventional HFC systems all digital transmissions between the home terminals and the ITS (CMTS or INA) head-end controller are encoded over the physical RF and fiber analog linear media by transmitting in both directions, over the HFC media, modulated analog passband fixed shape waveforms, called symbols, also as explained above. The fiber nodes are merely transparent linear points of conversion between the analog RF formats on the home side and the analog optical power formats on the head-end side. The digital detection (conversion of symbol waveforms into digital bits) of the upstream transmission occurs in the burst receivers at the head-end or hubs. Furthermore, the upstream digital communications multiple access (point to/from multipoint) is conducted between the ITS located at the HE or hubs and the HTs. Multiple upstream frequencies (FDMA) are used in the upstream, and on each upstream frequency a slotted- TDMA reservations based limited contention protocol allows multiple home terminals to transmit data bursts in coordination.
In accordance with the invention, the functionality of the ITS head-end controller is distributed between the head-end and the fiber nodes, moving the upstream detection from the head-end to the fiber nodes, i.e. placing the upstream burst receivers in the fiber nodes, transmitting the detected digital information in digital form from the fiber nodes to the head-end, and further introducing a novel distributed method to enable this by providing synchronization and calibration control between the two parts of the distributed ITS, which are now remote with respect to each other, namely the head-end MAC layer and the burst receivers in the fiber node.
One aspect in accordance with the invention transforms the analog return path from the fiber node to the head-end into a digital return path, by moving the digital burst receivers from their conventional location in the ITS head-end controller to the fiber node. With the digital burst receivers in the optical nodes, and the digital detection there, a digital link running over fiber optics is provided between the output of the burst receivers in the node and the ITS head-end controllers. This differs from conventional HFC systems which transmit analog passband waveforms encoding QPSK or 16-QAM symbols over the RF and fiber media, all the way from the home terminals to the head-end, and in which analog symbols are detected to digital form only at the head-end or hub.
First, in accordance with this invention the digital signals can be transmitted over part or the entire return path over the fiber media in passband modulation format such as ASK rather than baseband form as disclosed by Dail (referred to above). This is done in order to provide the additional advantage of compatibility with xe2x80x9clegacyxe2x80x9d systems which already use the upstream transmission band with proprietary modulation formats.
The reason to use passband ASK or equivalent passband modulation in the upstream is to meet a key objective of the proposed invention, namely the provision of a solution for backward compatibility with xe2x80x9clegacyxe2x80x9d networks (not of the DOCSIS or DVB type) wherein the lowband (e.g. 5 to 42 MHZ in the US) is already occupied in part by upstream transmissions from home terminals which do not confirm to the main standards DOCSIS or DVB that apply to the invention.
In contrast, Dail discloses baseband upstream transmission after the digital detection of the return path transmissions from the home terminals, which would preclude the linear transmission of the legacy spectrum back to the head-end and provide no solution for back compatibility.
In accordance with the invention, the ITS is synchronized with the remoted burst receivers into the node and with the home terminals, in a way which is compatible with the centralized RBLC-HFC based standards.
In these standards it is the role of the ITS at the head-end to provide a common time-base to all home terminals such that the system insures arrival of packets from each home terminal at the head-end, aligned on a time axis grid of regular time-slots. Distributing the functionality of the ITS between the head-end and the fiber node could conceptually be viewed as xe2x80x9celongating the wirexe2x80x9d from the burst receiver to the MAC layer of the CMTS/INA over as long a span as tens of Km (while possibly passing through layers of TDM multiplexing and demultiplexing on the way).
There is a technical problem, though, with this elongation of the link between the burst receivers and the MAC: the resulting spans are now indeterminate in length and could be possibly slowing varying due to temperature changes, or due to queuing delays in the TDM process. Unknown or varying propagation delays will violate a key requirement of the DOCSIS and DVB-RC standards to have fixed and known delays, and the ranging and synchronization protocols defined in the standards will fail, as these protocols are based on using synchronous reservations, whereby the transmissions from each home terminal are so timed as to fall within precise time-slots.
In accordance with this invention, further there is a distributed calibration and synchronization method and apparatus, without which a system as proposed by Dail would not function at all in HFC networks using multiple access standards or standards such as DOCSIS or DVB.
This failure is due to violation of the precise timing relationships between the times of transmission and reception, in the wake of the indeterminate delays.
The present system allows location of the RF to digital detection function in the nodes while maintaining full compatibility with existing (DOCSIS, DVB-RC) and possibly new RBLC-HFC standards. This is done by providing synchronization and calibration in the fiber node and applying minimum changes to the structure of existing head-end ITS systems, which changes may be still interpreted as conforming with the standards.
The result is that this system can function with standard DOCSIS cable modems or standard DVB set-top boxes or to be introduced home terminals of future RBLC-HFC standards, such as OpenCable.
The present system works with standard home terminals and keeps the multiple access protocols functional by providing distributed calibration and synchronization. Lack of such a solution in the prior art incapacitates the ability to have a functional system at all with DOCSIS and DVB home terminals.
The present distributed calibration and synchronization involves the remoted digital burst receivers and the head-end MAC. This is implemented by incorporating in the optical node, along with the burst receivers, an additional stripped-down home terminal, making use of its downstream cable demodulator plus MAC layer as further indicated below.
In one embodiment this is done by introducing into the node either a cable modem assembly, in the case of a network home cable modems, or a set-top assembly, in the case of a network with home set-tops, and modifying the software run by the cable modem or set-top introduced at the node.